The present invention relates to a very high vacuum cooling trap. Accompanying the progress in the technique for establishing very high vacuums, cooling traps have been widely used. Examples include a liquid nitrogen shroud for an MBE apparatus, and cooling traps for vacuums such as a trap for sublimation, a trap for oil diffusion pumps, and the like. Many instruments for measuring high vacuum have also been equipped with a shroud. Further, cryopumps which use liquid helium also utilize the trap effect, as a matter of course.
Shroud traps are generally made of stainless steel. Some traps such as those for cryopumps have been equipped with cooling fins. In most of them, however, a cooling medium such as liquid nitrogen is stored in a stainless steel vessel, and the stainless steel vessel is cooled by the cooling medium so that gaseous molecules are adsorbed. With such vacuum cooling traps employing stainless steel, however, strong and heavy support rods must be used to support the traps, resulting in increased heat conductivity, and consuming the cooling medium in large amounts.
The amount of liquid nitrogen which is consumed in the initial filling is determined by [weight of trap].times.[latent heat]. Here, SUS:Aluminum=2:1; hence, aluminum is advantageous. SUS designates stainless steel.
Because of the fitting type, the portions filled with the cooling medium exhibit the cooling trap effect, but the unfilled portions do not exhibit that effect. Therefore, it is necessary to supply the cooling medium while monitoring the liquid level at all times. Since the cooling traps are produced by welding, fine cracks exist in the welded portions. Therefore, water which has infiltrated through the cracks, freezes, thereby further cracking the welded portions and causing leaks. When holes are formed in the cooling trap or when a cooling trap structure is complex, the welding structure is also very complex. Further, a cooling trap of the fitting type is constructed as a unified structure by welding. Accordingly, such a cooling trap cannot cope with thermal expansion or contraction.